1. Field of the Invention
The present invention relates to an over-coated optical fiber used for an optical fiber cable.
2. Description of the Related Art
As such an over-coated optical fiber used for an optical fiber cable, there are generally used a single-fiber over-coated optical fiber in which an over-coating is applied onto an optical fiber element, and a ribbon over-coated optical fiber in which optical fiber elements are arranged in parallel and an over-coating is applied to the optical fiber elements in the lump.
The latter of them, that is, the ribbon over-coated optical fiber has such a sectional structure as shown in FIGS. 2A and 2B. FIG. 2A shows a four-fiber over-coated optical fiber, and FIG. 2B shows an eight-fiber over-coated optical fiber. As shown in FIG. 2A, each optical fiber element 12 is constituted by a glass fiber 11a having silica as its main ingredient and a coating 11b of UV curable resin applied onto the glass fiber 11a. A four-fiber over-coated optical fiber 15 is constituted by four optical fiber elements 12 arranged in parallel and covered with an over-coating 13 of UV curable resin. On the other hand, as shown in FIG. 2B, an eight-fiber over-coated optical fiber 16 is obtained in a manner so that two sets of optical fiber arrangement in each of which four optical fiber elements 12 are arranged in parallel and covered with an over-coating 13, are further arranged in parallel and an over-coating 14 of UV curable resin is applied onto the two sets.
These over-coated optical-fibers are used mainly for optical fiber cables having such a structure as follows. FIG. 3 shows a multi-fiber optical fiber cable which has been widely used. In the multi-fiber optical fiber cable, a spacer 18 is formed by extrusion molding with a plastic material such as polyethylene, or the like, so that it has spiral grooves 18a formed around a tensile core 17 of a steel wire, a steel stranded wire, FRP or the like. Then, sets of four-fiber over-coated optical fibers 15 are received in each of the grooves 18a while being laid on each other. Finally, an upper winding 19 and a plastic jacket 20 are provided on the outer side of the spacer 18. The number of the grooves are determined in accordance with necessity, and it is not limited to five in such a case as shown in FIG. 3.
In addition, the spiral direction of the spiral grooves 18a provided in the spacer 18 may be unidirectional, or may be inversional in the longitudinal direction.
The over-coated optical fiber shown in FIG. 4 is a few-fiber optical fiber cable mainly used in the place where there is a small demand of subscribers. This optical fiber is configured so that an eight-fiber over-coated optical fiber 16 is received in a pipe 21 of a plastic material while being twisted at a pitch in the longitudinal direction, and a plastic coating 23 having tensile cores 22 of steel wire buried therein is provided on the outside of the pipe 21. A jelly mixture may be charged into a space 24 in the pipe 21 in order to ensure watertightness.
Generally, when an over-coated optical fiber is produced by using one or more optical fiber elements so that an optical fiber cable is manufactured by using a plurality of such over-coated optical fibers, tension is given to the over-coated optical fiber. Further, when the optical fiber cable is laid in a conduit line or the like, tension is further given to the optical fiber cable. The tension given to the optical fiber cable may be transmitted to the optical fiber elements in accordance with the structure of the optical fiber cable. The tension causes extensional strain in the optical fiber cable, the over-coated optical fibers, and the glass fibers.
The glass fibers in the over-coated optical fibers experience the extensional strain generated at the time of manufacturing the over-coated fibers and the optical fiber cable and the extensional strain due to the cable tension at the time of installation of the cable. The cable tension during installation is usually released after laying, therefore, the extension strain generated at the time of manufacturing remains in the glass fibers for a long-term use of the optical fiber cable.
For example, in the case of the optical fiber cable shown in FIG. 3, usually the individual over-coated optical fibers 15 must be arranged in each of the spiral grooves 18a so as not to be loosened when the over-coated optical fibers 15 are received while being laid on each other in each of the spiral grooves 18a. Therefore, the over-coated optical fibers 15 are received while being given tension, so that extensional strain remains in the respective glass fibers in the over-coated optical fibers 15.
On the other hand, in the optical fiber cable in FIG. 4, the over-coated optical fiber is received in the pipe while being twisted spirally at a pitch of, for example, about 250 mm so as to endure bending of the cable. Therefore, extensional strain is produced, because of twisting, in optical fiber elements in a position near the end of the ribbon in comparison with optical fiber elements disposed at the center of the ribbon. Extensional strain is produced also in glass fibers substantially to the same extent.
In order to make optical fiber elements endure tension and extensional strain given thereto at the time of laying the over-coated optical fiber, a screening step of giving a constant tension to the optical fiber elements and removing portions not-proof against the tension is given in the process of manufacturing the optical fiber elements. Therefore, there is a relationship that the manufacturing yield becomes smaller if this tension set in the screening step is made larger, while the manufacturing yield becomes larger if this tension set in the screening step is made smaller. Accordingly, the tension set in the screening step has an influence on the manufacturing cost.
On the other hand, it is known that the breaking endurance of the glass fiber is reduced if extensional strain is continuously given to a glass fiber for a long time, and there is a correlation between the residual extensional strain and the endurance of the glass fiber. In addition, it is known that there is another correlation between the endurance of the glass fiber and the tension set in the screening step at the time of manufacturing optical fiber elements, and if the tension set in the screening step is increased, the endurance of glass fiber is prolonged.
From those facts, when a certain goal is given to the endurance of an optical fiber cable, the larger the extent of the extensional strain residual is in glass fibers after laying of the optical fiber cable, the larger the tension set must be made in the screening step. Therefore, if the extensional strain residual in the glass fibers can be reduced, the tension set in the screening step can be reduced. Then, the manufacturing yield of the optical fiber elements can be increased correspondingly, and the manufacturing cost can be reduced.
It is an object of the present invention to solve the foregoing problem in the conventional techniques, and to provide an over-coated optical fiber in which tension set in the screening step can be reduced.
According to the present invention, provided is an over-coated optical fiber in which one or more optical fiber elements each constituted by a coated glass fiber are arranged, and an over-coating is applied to the outer circumference of the arrangement of the one or more optical fiber elements. In the over-coated optical fiber is, the glass fibers have compression strain so as to be compressed in the longitudinal direction.
Further, in a method of manufacturing an over-coated optical fiber according to the present invention, the feeding speed of optical fiber elements to an over-coating device is made larger than the take-up speed of the over-coated optical fiber to thereby give compression strain to glass fibers so as to compress the glass fibers in the longitudinal direction.